The inhibitor of apoptosis protein Survivin is overexpressed in most cancers and hematologic malignancies and is an indicator of aggressive disease and poor outcome. Survivin is widely expressed during development but not in most adult tissues, making it an attractive therapeutic target. We were the first to demonstrate that Survivin is expressed and growth factor regulated in normal hematopoietic stem cells (HSC). New evidence from several areas now supports a critical role in preservation of cell viability and maintenance of normal mitosis. This leads to 2 hypotheses. First, Survivin is a normal regulator of HSC and Survivin targeted therapies may have hematologic toxicities. We will modulate Survivin expression in adult mouse HSC using retroviruses and quantitate its effects on hematopoiesis by competitive repopulation following transplantation and its role as a potential oncogene. Second, Survivin is a critical and multifaceted mediator of the cell death versus cell division decisions in HSC and understanding Survivin pathways will identify new therapeutic targets. We will investigate signaling pathways that regulate Survivin production and the molecular/biochemical relationships between Survivin and the intracellular regulators of apoptosis and cell cycle, i.e., cyclins, cyclin dependent kinases (cdks), cdk inhibitors (CDKIs), and the p53 family of tumor suppressors. We will investigate these interactions in normal primary HSC, in novel hematopoietic and nonhematopoietic cell line models, in which Survivin expression/disruption can be transiently or conditionally modulated, and in mice, in which genes for cell cycle and apoptosis related proteins have been deleted. Understanding the hematologic consequence of normal and abnormal Survivin expression and defining the molecular mechanisms of act of Survivin in regulating cell survival, cell cycle progression, and cell division in normal HSC will provide important information on normal cell survival and cell cycle regulation critical to the design of safe and efficacious Survivin-based therapies that target cell cycle and apoptosis.